You can find lots of different gauges of hemp twine on Amazon with different colours.
Cotton yarn for dishcloths and things is an option closer to your original idea.
I agree with everything @FloorCandy says. If you use a natural fiber they will be more durable and will probably carry the scent through better as well.
Buy buy a few balls on sale to see how fast you go through it. You will be able to make 50 of those toys from a few small balls.
And trust...